1. Field of the Invention
Embodiments disclosed herein relate generally to drill bits and other cutting tools. In particular, embodiments disclosed herein relate to PDC drill bits having diamond shearing elements.
2. Background Art
Historically, there have been two main types of drill bits used for drilling earth formations, drag bits and roller cone bits. The term “drag bits” refers to those rotary drill bits with no moving elements. Drag bits include those having cutting elements attached to the bit body, which predominantly cut the formation by a shearing action. Roller cone bits include one or more roller cones rotatably mounted to the bit body. These roller cones have a plurality of cutting elements attached thereto that crush, gouge, and scrape rock at the bottom of a hole being drilled.
Drag bits, often referred to as “fixed cutter drill bits,” include bits that have cutting elements attached to the bit body, which may be a steel bit body or a matrix bit body formed from a matrix material such as tungsten carbide surrounded by a binder material. Drag bits may generally be defined as bits that have no moving parts. However, there are different types and methods of forming drag bits that are known in the art. For example, drag bits having abrasive material, such as diamond, impregnated into the surface of the material which forms the bit body are commonly referred to as “impreg” bits. Drag bits having cylindrical cutting elements made of an ultra hard cutting surface layer or “table” (typically made of polycrystalline diamond material or polycrystalline boron nitride material) deposited onto or otherwise bonded to a substrate are known in the art as polycrystalline diamond compact (“PDC”) bits. The cutting element substrate provides a way for the ultra hard cutting table to be attached to the drill bit. In particular, the substrate material is generally capable of allowing strong and secure attachment of the cutting element to the drill bit. While the substrate allows for attachment of the ultra hard cutting table to the bit, the use of the substrate tends to place a limit on the thickness of the ultra hard cutting table that is feasible without excessive stresses between the two bodies or excessive risk of delamination of the ultra hard cutting table.
An example of a conventional PDC bit having a plurality of cutters with ultra hard cutting tables is shown in FIG. 1. The drill bit 100 includes a bit body 110 having a threaded upper pin end 111 and a cutter face 112. The cutter face 112 typically includes a plurality of ribs or blades 120 arranged about the rotational axis L of the drill bit and extending radially outward from the bit body 110. Cutting elements, or cutters, 150 are embedded in the blades 120 at predetermined angular orientations and radial locations relative to a working surface and with a desired back rake angle and side rake angle against a formation to be drilled. Cutters 150 are conventionally attached to a drill bit or other downhole tool by a brazing process so that the ultra hard cutting table faces into the direction of rotation of the bit. In the brazing process, a braze material is positioned between the cutter substrate and the cutter pocket. The material is melted and, upon subsequent solidification, bonds (attaches) the cutter in the cutter pocket.
A plurality of orifices 116 are positioned on the bit body 110 in the areas between the blades 120, which may be referred to as “gaps” or “fluid courses.” The orifices 160 are commonly adapted to accept nozzles. The orifices 160 allow drilling fluid to be discharged through the bit in selected directions and at selected rates of flow between the blades 120 for lubricating and cooling the drill bit 100, the blades 120 and the cutters 150. The drilling fluid also cleans and removes the cuttings as the drill bit 100 rotates and penetrates the geological formation. Without proper flow characteristics, insufficient cooling of the cutters 150 may result in cutter failure during drilling operations. The fluid courses are positioned to provide additional flow channels for drilling fluid and to provide a passage for formation cuttings to travel past the drill bit 100 toward the surface of a wellbore (not shown).
Cutting elements commonly used with PDC drill bits may be formed by placing a mixture of diamond particles and catalyst material adjacent to a substrate (typically a carbide substrate) and sintering the assembly, or by providing the catalyst material from the adjacent substrate, wherein the catalyst infiltrates and bonds together the diamond particles to form a polycrystalline diamond layer attached to the substrate. Alternatively, a mixture of a catalyst material and diamond crystals may be placed in a pressure vessel without a substrate and sintered together to form a polycrystalline diamond layer without an attached substrate. The polycrystalline diamond layer may then be immersed in a leaching agent to leach the catalyst material remaining between the bonded together diamond crystals, thereby forming a thermally stable polycrystalline diamond layer.
A significant factor in determining the longevity of PDC cutters is the generation of heat at the cutter contact point with a rock or earth formation, specifically at the exposed part of the PDC layer, caused by friction between the PCD and the formation. This heat causes thermal damage to the PCD in the form of cracks (due to differences in thermal expansion coefficients) which lead to spalling of the polycrystalline diamond layer, delimitation between the polycrystalline diamond and substrate, and back conversion of the diamond to graphite causing rapid abrasive wear. Thermal exposure to the PCD may also occur during brazing of the cutting elements onto the drill bit or other cutting tool. Selection of braze materials depends on their respective melting temperatures, to avoid excessive thermal exposure (and thermal damage) to the diamond layer prior to the bit (and cutter) even being used in a drilling operation.
Conventional polycrystalline diamond is stable at temperatures of up to 700° C., after which observed increases in temperature may result in permanent damage to and structural failure of polycrystalline diamond. This deterioration in polycrystalline diamond is due to the significant difference in the coefficient of thermal expansion of the binder material, e.g. cobalt, as compared to diamond. Upon heating of polycrystalline diamond, the binder material and the diamond lattice will expand at different rates, which may cause cracks to form in the diamond lattice structure and result in deterioration of the polycrystalline diamond. However, thermal fatigue does not only occur at temperatures above 700° C. Rather, the differential expansion (between the binder material and diamond) even occurs at temperatures as low as 300-400° C., still causing thermal fatigue in the diamond body. Further, damage to polycrystalline diamond can also result from the loss of some diamond-to-diamond bonds (from the initiation of a graphitization process) leading to loss of microstructural integrity and strength loss.
In order to overcome this problem, strong acids may be used to “leach” the cobalt from the diamond lattice structure (either a thin volume or entire table) to at least reduce the damage experienced from heating diamond-cobalt composite at different rates upon heating. Examples of “leaching” processes can be found, for example, in U.S. Pat. Nos. 4,288,248 and 4,104,344. Briefly, a strong acid, typically nitric acid or combinations of several strong acids (such as nitric and hydrofluoric acid) may be used to treat the diamond table, removing at least a portion of the co-catalyst from the PDC composite. By leaching out the cobalt, thermally stable polycrystalline (TSP) diamond may be formed. In certain embodiments, only a select portion of a diamond composite is leached, in order to gain thermal stability without losing impact resistance. As used herein, the term TSP includes both of the above (i.e., partially and completely leached) compounds. Interstitial volumes remaining after leaching may be reduced by either furthering consolidation or by filling the volume with a secondary material, such as by processes known in the hart and described in U.S. Pat. No. 5,127,923, which is herein incorporated by reference in its entirety.
Additionally, the design of conventional PDC cutters often results in failure due to wear and/or chipping of the diamond layer. FIGS. 2A and 2B show examples of the failure modes experienced by conventional PDC cutters brazed in a cutter pocket. As shown, chipping and/or wear of the diamond layer particularly affects the part of the diamond layer that is positioned to contact the borehole and the interface region between the diamond layer and substrate. For example, the circled cutter in FIG. 2A has chipping at the part of the diamond layer that contacts the borehole and the circled cutter in FIG. 2B has chipping at the interface region. FIG. 2B also shows a PDC cutter having a wear flat 200 formed at the part of the diamond layer that contacts the borehole. The wear flat 200 has a chord length of about 0.300 inches. As wear flats increase in size, the exposure of the diamond/substrate interface to the cutting action increases, which may lead to more severe failures. FIG. 3 shows a progression of a wear flat 300 forming in the diamond layer 312, interface region 314, and substrate 316 of a conventional PDC cutting element 310. As the chord 309 of the wear flat 300 grows, it increases in size from covering only part of the diamond layer 301, 302, 303 to covering part of the diamond layer, interface region, and substrate 304, 305, 306, 307, 308.